1. Marine Structure (FIG. 1)
A marine structure means a structure having no component connected to the land and capable of staying at one point on the sea under any weather condition. The marine structure is used for various purposes. The marine structure may be installed for developing or producing submarine oil or gas fields or generating new regeneration energy (e.g., wind power, tidal power, wave power or the like) and may also be used as a port structure for a large oil tanker to come alongside the pier. In order to anchor a large oil tanker, a sufficient water level is required. Therefore, if dredging is not available, jetties and dolphins are extended to a deep point to build a port. This is a so-called offshore marine terminal. Recently, marine structures are often installed for power generators, bulk storage plants, fishing bases or the like.
Seeing materials of a marine structure, steel is used most. At this time, a portion submerging into the sea is made of a steel pipe with a circular section. The circular sectional shape receives less force from waves or sea currents, and a steel pipe pile may be used as a foundation. In addition, buoyancy may be applied thereto when a structure is installed.
An upper structure above the sea is made of shape steels such as H-beams which ensures easy fabrication and maintenance. Steel products are easily corroded and sessile marine lives easily inhabit thereat. In spite of such drawbacks, however, steel products are most widely used as a material of the marine structure due to easy fabrication and installation, clear design, stiff structure or the like.
In an area having a deep sea level and hard seabed topography, a concrete structure is frequently used. Concrete has good corrosion-resistance and may maintain a stable state just by its weight. A huge concrete cylinder may be used as a storage facility for storing oil or the like and may be easily carried and installed while ensuring easy structure inspection. However, such a concrete structure is limitedly used in the North Sea or polar regions since it is not easily fabricated and also limited to seabed topography conditions. The marine structures may be classified into three groups depending on their natures.
(1) Floating-Type Structure
The floating-type structure has been mainly used for oil drilling, and recently also used for ocean power plants, oil storage facilities or the like. A floating-type structure for oil drilling should ensure both mobility and fixedness. In other words, when oil drilling is failed, the floating-type structure can be instantly moved to another region, and during oil drilling, the floating-type structure should be firmly fixed not to give an excessive force to an oil drilling pipe. Three kinds of floating-type oil drilling ships are representatively used in the art.
a. Drill Ship (FIG. 2)
A drill ship has a self-propulsive function to ensure mobility, but its fixedness is ensured by means of mooring or dynamic positioning, which causes rolling or pitching under a bad weather to cause hardship on its operation.
b. Jack-Up Rig (FIG. 3)
In operation, three legs are fixed to the seabed to ensure stability. If oil drilling is finished, the legs are lifted up by means of a jack-up method so that the jack-up rig floats by the buoyancy of the hull. At this time, a draft line is used for towing so that the jack-up rig is moved to another region. The jack-up rig ensures better fixedness in comparison to a drill ship, but it cannot operate at a deep area and does not ensure good mobility. In addition, under a bad weather, the jack-up operation may be temporarily interrupted.
c. Semisubmersible Oil Drilling Ship (FIG. 4)
A semisubmersible oil drilling ship is a floating structure having four or six legs, and horizontal members, so-called pontoons, for connecting the legs create buoyancy. The semisubmersible oil drilling ship is structurally stable but may come into an unstable state at a bad weather since it has a great upper deck area and is likely to contain a lot of equipment thereon. For this reason, a rollover accident has been occurred. The semisubmersible oil drilling ship also requires great fabrication and operation costs since it does not have a self-propulsive function.
d. FPSO (Floating Production Storage Offloading) (FIG. 5)
FPSO is a floating production storage and offloading unit, which is a special ship suitable for small-sized deep oilfield development since it may allow loading and unloading as well as crude oil mining and also ensure free movement. Along with the coming presence of higher oil price, oil producers in various countries over the world greatly invest in oil exploration development projects on the assumption that the oil price increases further. Accordingly, a new-type FPSO having economic feasibility at offshore oil fields and ensuring convenient carriage, different from an existing fixed oil drilling ship, has appeared.
An overall figure of the FPSO is similar to a general supertanker. However, equipment required for oil purification, gas compression, crude oil unloading, sea water injection, self-power generation or the like is installed at its upper portion to ensure crude oil mining, purification, storage and unloading by itself.
Recently, it is attempted to build nuclear power plants, gas turbines, wave power plants, tidal power plants, see breeze power plants, solar power plants, waste incinerators or the like on a floating structure on the sea since it becomes more difficult to choose a site therefor due to environmental pollution and NIMBY phenomenon. In addition, an offshore airport formed as a large steel structure to ensure taking-off and landing for 24 hours a day is still remained as a long-term project. A lot of small- or medium-sized diesel power plants have been installed and operated on barges, and seawater desalination plants and oil and gas storage facilities are also installed as floating facilities.
(2) Fixed Structure
A steel pipe welded structure, so-called a jacket, is a fixed marine structure most frequently used in the art (FIG. 6). This structure is generally fabricated on the ground, loaded on a barge, carried to a desired region and then launched thereat. At this time, piles are driven by means of four to eight legs. Main facilities at an upper portion are supported by these piles, and a steel pipe structure supports these piles laterally by means of legs and braces so that the piles may incorporate against to a lateral force.
The jacket is named since this structure surrounds piles. A pile is driven into a depth of about 100 meters at the seabed to permanently fix an offshore platform in the seabed topography and transfers lateral and vertical loads to the seabed to stably maintain the structure.
Main facilities at the upper portion are composed of a structure having two or three decks, and in an offshore complex having several platforms, bridges are installed thereat to connect the platforms. The jacket platform generally has a life span of about 20 years and is widely used for oil producing and drilling at the seabed and residence on the sea.
A concrete gravity based structure (GBS), which is a kind of a fixed structure, has a supporting force by its weight, instead of piles, against an external load (FIG. 7). At this time, a stable and hard seabed ground is required in order to prevent subsidence at the gravity based structure for a long time.
A mono-tower concrete plate form having a great base is sometime installed in a polar region in order to decrease a risk of collision against an iceberg or in a deep sea having a hard seabed ground for a better economic design. In a shallow sea, an artificial island is formed by reclamation to make a marine city, an offshore airport, an oil production facility or the like.
(3) Mobile Structure
A mobile structure is a kind of floating-type structure, but a steel wire is connected from a fixed structure installed on the seabed to induce lateral stability of a floating structure. This structure has been designed to make an effort to install an economic structure in a deep sea. Guyed towers, tension leg platforms (TLP) or the like are regarded as such a mobile structure.
In the guyed tower, a vertical load of the platform is supported by a steel structure extending vertically to the seabed surface with no slant, and a lateral load is supported by steel wires connected in four directions to the steel structure and fixed to the seabed surface.
The TLP is a structure in which a steel wire (a tendon or tether) is connected vertically from a leg at each corner to the fixed structure at the seabed surface to cover a lateral load to some extent. Buoyancy of the upper platform constantly maintains the tension of the steel wires, and thus vertical movement of the platform is attenuated, which gives stability advantageous for deep sea oil well development. If the work at any place is completed, the TLP may be moved to another place and installed again, which is very advantageous when developing an oil well with a small oil resource. At earlier stages, the TLP was made of steel materials, but the upper structure and the seabed structure are gradually replaced with concrete to be used as a temporary oil storage facility.
In addition, as a deep seabed structure, there is also proposed an articulated tower in which an upper portion made in a jacket-type steel structure and a lower portion made with a concrete case are connected to each other, or an upper portion made of a concrete floating structure and a lower portion made of a steel truss are connected with special joints to remove a bending force.
2. Natural Conditions to be Considered in Designing a Marine Structure
(1) Water Level and Seabed Topography
A water level represents a vertical distance from a tidal datum till the seabed surface. Here, the tidal datum means a minimal low-water level, and the sea level rarely lowers below the tidal datum if ever. Accurate measurement of the water level and proper understanding on irregularity of the seabed topography of the region are a trigger of a marine structure design, and from them, it is possible to determine a height of the marine structure, a shape of the lower portion of the structure, a vertical location of a ship berth facility, a range of anti-corrosion design or the like and to verify geological stability of the structure. For continuous measurement of the water level, a precision depth recorder serving as a sound fathometer, a side scan sonar allowing two-dimensional understanding, or the like are used.
(2) Seabed Topography
Physical and engineering understanding on the seabed topography which supports the base of the marine structure is essential to design an economic and safe structure. The seabed topography is investigated to analyze topography of the seabed surface and the seabed stratum till a lower bed rock. The seabed topography is generally investigated by obtaining a continuous topography sampling by direct boring and analyzing the same at a laboratory to collect design data.
However, as a preprocess of the boring, seabed topography information around the structure should be collected using geophysical equipment such as a subbottom profiler, a boomer, a sparker, an air gun or the like. At this time, for more substantial understanding on a swallow stratum, samples may be also collected using a piston driller, a grab sampler or the like. This is because seabed topographical characteristics of the seas around an installation spot of the structure should be figured out in advance to determine a main boring point, and topographical states of other non-boring regions should also be checked. If any unusual structure in a fault or sedimentary stratum, an abrupt change of the seabed topography, abnormal erosion, a stream of sediments or the like is found in the seas around the structure, a serious problem may occur on the stability of the structure.
After geophysical inspection data are analyzed, in consideration of the degree of stratum change of the surroundings, the shape, importance and number of marine structures are determined. The drilled samples give basic data for figuring out various soil characteristics as well as stress factor and displacement of piles through field analysis and laboratory analysis to make a foundation design. In particular, a stratum near the seabed surface should be analyzed intensively since soil at the stratum gives a great influence in calculating settlement, allowable bearing capacity, horizontal displacement or the like of the structure.
(3) See Breeze
Wind applies pressure or causes vibrations to an upper structure or facilities on the sea surface, thereby giving an influence thereto. The intensity of wind is trivial in comparison to the wave or sea current, but the wind should not be ignored since it gives a great moment arm from the seabed surface.
The wind on the sea surface may be classified into sudden gust and continuous wind. The sudden gust is wind whose wind direction and wind speed generally continue less than 1 minute, and the continuous wind is wind whose wind direction and wind speed generally continue over 1 minute. A wind speed of the continuous wind is used for foundation design of a marine structure, and the sudden gust is applied in designing each unit facility and a small structure which is sensitive to wind.
At a deep sea guyed tower or a tension leg platform having a long natural period, the wind speed spectrum should be used in order to consider a dynamic effect according to the natural period.
(4) Wave
In designing a marine structure, the sea wave gives a greatest influence. The sea wave most directly gives a great force to the foundation design or each component of the structure and thus serves as a critical element in designing sizes or lengths of components.
The most important characteristic of the wave is irregularity. For this reason, a spectrum model becomes a barometer representing any sea state, and at this the structure should also be analyzed in a statistical way. However, in the light of convenient designing as well as reason and experience, a regular wave modeling is also considered as being very suitable for designing a marine structure. The regular wave defines a wave to have a series of waveform with a constant wavelength, a constant wave height and a constant wave period. The regular wave models available in the art includes an airy wave, a fifth-order stoke wave, a stream function or the like.
An applied wave model depends on a water level, a structure shape, an applied wave height or the like. The wave selected as above is called a design wave, and the design wave has three variables, namely a wave height, a wave period and a waver level. From the design wave, velocity and acceleration of water particles applied to each point of each member or the structure are calculated to select a final wave power from the Morrison equation.
Wave is generated due to various reasons, particularly wind, and for this reason, in designing a structure, wind and wave are applied in the same direction to obtain a maximum design external force. In addition, if sea wave data for a reasonable period are present about the installation region of the marine structure, the design wave may be obtained without a special difficulty. However, in many cases, there is present only wind speed data, and thus various methods for calculating a design wave from the wind speed have been developed. At this time, a cautious wave height and a mean wave period are obtained in a statistical way in consideration of a repetition period, and a maximum wave height (a design wave height) and a wave period corresponding thereto are calculated therefrom.
(5) Sea Current
If wave has a stream with a waveform formed by vibrations of water particles, the sea current may be regarded as a stream in which water particles directly move in a horizontal direction due to various factors. Therefore, if this stream meets the structure, a constant horizontal force is applied to the structure, and when a ship approaches the marine structure for berthing, the sea current gives a constant influence on the ship.
Factors of generating a sea current may be classified into large-scale factors and local factors. The large-scale factors may include a constant wind, rotation of the earth, difference in temperatures or salinities or the like, and the local factors may include sediments on the seabed, wave, tide, wind, typhoon or the like. The velocity of water particles by the sea current is added as a vector sum to the velocity of water particles by the sea wave to form an overall force applied to the structure.
(6) Tide
Among influences applied by movement of celestial bodies, the tide outstands most. A flood time caused when the gravitations of the moon and the sun are added and an ebb tide caused in a reverse case are familiar sea movements acquainted by anyone through experiments. However, ascending and descending of the sea level is not caused only by celestial bodies, but the phenomenon locally caused by wind, wave, or difference in pressures should not be ignored. Therefore, the maximum design water level is determined in consideration of all factors as above.
If the structure is close to the coast or located in a closed island sea such as a bay, the ascending and descending effect of tide or the like becomes more conspicuous. Therefore, if this effect is not suitably considered in designing, a serious problem may be caused. Generally, an external force should be calculated and the height of the deck should be determined based on the case in which a maximum wave height applies to the structure at the maximum water level. In addition, a vertical range of a maximum water level and a minimum water level is calculated, and facilities for anchoring a ship should be installed according to the vertical range. Moreover, in case of a steel structure, the vertical range should be applied in selecting a maximum corrosion range, a thickness of sessile marine lives.
(7) Seabed Earthquake
An aseismic design is essential for a marine structure, and if the marine structure is dynamically sensitive, a dynamic analysis by earthquake must be accompanied. In case of a structure with a high importance or an extra-large structure, a lower topography structure is thoroughly investigated to consider dislocation or sediment transfer which may occur all-at-once by earthquake.
(8) Marine Bodies
As time goes, marine bodies stick to the marine structure and grow thereat. As the marine bodies grow to have a thickness of 2 to 3 centimeters, an area and volume of each component to which wave or sea current is applied increases rapidly. In addition, the external surface of each component becomes rougher to increase a drag effect, and in case of a steel material, corrosion may be locally promoted. Therefore, in designing, the effect of marine bodies must be considered. Meanwhile, as sessile marine lives cover the surface of the structure, it becomes more difficult to maintain and manage the marine structure, and thus it is required to partially remove the marine bodies.
(9) Others
In addition to the above, density and salinity of sea water associated with corrosion of a steel marine structure and properties of sea water, an abrupt change of sea water temperature according to depth, a hydrostatic pressure increasing 1 atmospheric pressure per 10 meters or the like are natural conditions which should be basically considered in designing. Moreover, instability of the seabed ground, which may be caused by sea wave, seabed earthquake, rapid sediment or the like, as well as scouring and deposition, which may occur around the marine structure foundation by constant sea current or sea wave, should also be addressed in the foundation design.
3. Mooring System of a Marine Structure
A mooring system of a marine structure includes three types, namely a single-point mooring system, a multi-point mooring system and a dynamic positioning system.
(1) Single-Point Mooring System (FIG. 8)
The single-point mooring system is frequently used for unloading oil, and particularly widely used in a deep sea level instead of a fixed structure. The fixed structure needs a small maintenance cost and has a high operation rate in the oil unloading work, while the single-point mooring system needs more maintenance cost but has a low early stage investment. The single-point mooring system has a following structure.
a. CALM (catenary anchor leg mooring) type: a structure is moored to mooring cables from a multi-point mooring buoy. Most of them are single-point mooring systems and suitable for a water level not greater than several ten meters.
b. SALM (single anchor leg mooring) type: this moors to a structure having an agitating-column buoy. This type is suitable for a water level of several ten to several hundred meters.
c. Yoke type: a structure is moored from a CALM type or SALM type buoy.
d. Turret type: a cylinder coupled to a rotating machine at the center of the structure is moored at multi points such as the CALM-type buoy.
(2) Multi-Point Mooring System
The multi-point mooring system is a mooring system for maintaining a marine structure at a certain location accurately and preparing a great mooring force, and is adapted to an ocean work vessel, an oil drilling rig or the like.
The mooring cable may be arranged in various patterns. The mooring cable uses a wire rope and a chain, or an intermediate sinker or buoy is installed to stabilize the mooring cable. An anchor is subject to a horizontal force (having a tangential angle θ=0 at the seabed surface), but the sinker is subject to a horizontal force and a vertical force (an access angle θ>0).
(3) Dynamic Positioning System (FIG. 9)
Mooring using a mooring cable is limited to a water level, and thus there are not many records in a water level over several hundred meters even for an oil drilling rig. In order to maintain a marine structure at a constant location without using a mooring cable, a dynamic positioning system is used. A location of the marine structure is detected by using a global positioning system (GPS), and a screw and a thruster are operated by calculating an operation amount required for maintaining the marine structure at a predetermined location.
The quiescence of the dynamic positioning system is expressed as a ratio (%) of a horizontal movement amount with respect to a water level, and about 1% at a water level of about 100 meters. Along with an increase of the water level, the ratio also increases. In particular, in a marine structure having a riser, the ratio has a limit of about 5%, and if the ratio becomes 10%, the riser may be bent or broken. For this reason, the quiescence of the dynamic positioning system should be carefully maintained. The location of the marine structure may be maintained by the mooring system in a water level over several hundred meters, but as the water level increases, the dynamic positioning system may be more advantageously used.
4. Stable Mooring of a Marine Structure
As described above, the marine structure may float on the ocean and function to product, store and/or unload liquefied gas. In particular, a floating marine structure such as LNG FPOS is a ship for a complicated function by mounting facilities for producing (or, collecting) natural gas at the ocean and then liquefying and storing the same, thereby decreasing the necessity of a land liquefying and storing facility which consumes enormous expends.
A floating marine structure such as LNG FPOS has a rotary turret, and a turret and an anchor of the seabed may be connected to mooring lines to moor the marine structure on the ocean. The rotary turret is fixed by the mooring line and the anchor, but the marine structure may operate properly at a fixed location on the ocean in spite of wave since its hull may move in a rotating direction based on the rotary turret.
Here, if the mooring line and the anchor are broken beyond an allowable level of the system, a flexible riser connecting the single point mooring (SPM) and the pipe line end manifold (PLEM) is damaged, which results in spillage of high-pressure and high-temperature crude oil. Such an oil spillage accident causes huge economical, life and environmental losses. In addition, as the concern on environments rises over the world, the damage of SPM should be prevented in advance.
Meanwhile, design and interpretation of mooring devices which have been installed and used until now have totally depended on foreign technology. In fact, a mooring device is not designed suitably for an environment of an installation region, and an interpretation program for mooring devices is imported from foreign countries, which consumes enormous foreign currency. Therefore, it is required to make a long-term plan to actively and aggressively secure technical skills for developing marine resources continually, and thus to enhance the import substitution effect.
In addition, a tension applied to each mooring line of a marine structure is not regular but changes continuously due to a loading amount, a change of tide, or a difference between the ebb and flow. Moreover, if cargos are loaded to or unloaded from a ship while a floating marine structure is being moored, the ship may be dipped more or less due to a difference in buoyancy according to the change of loaded cargos. In this case, the tension applied to the mooring line also continuously changes accordingly.
Therefore, since the tension of the mooring line of the marine structure changes continuously, a worker should frequently monitor whether an excessive tension is applied to a specific rope, and in order to suitably distribute the tension, the mooring line should be tightened or loosened appropriately.
In addition, in an existing art, the degree of tension applied to a mooring line is judged based on experience or naked eye of a worker. However, a tension monitoring system has recently introduced to large ships such as an oil tanker and a gas carrier so that tensions applied to a plurality of mooring lines are monitored through a monitoring computer installed at a control center.
Moreover, in order to exactly analyze an accurate location, behavior and stability of two articles floating on the ocean by using a real-time data management system and then predict and alarm an emergent situation, sensors for sensing marine environments and behavior states should be developed, installed and operated. In particular, a next-generation mooring system for material development, behavior analysis, installation technique, operating technique, system management or the like should be developed to make a perfect mooring system.